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Air Arms (4##/5##)- Terry Robb Firing Valve

Discussion in 'Technical' started by Virak, May 18, 2020.

  1. Virak

    Virak Busy Member

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    At last I have recieved the above firing valve,it is likely at soonest it will be in place next weekend as time and the need to work out why the hex bolt on the firing bolt has a scored head( manufactured this way or wear see image - the bright bit on lower right of hex bolt)
    However I have attached a few images.
    The prime difference is the sealing face is concave inboard of the contact point.
    To me the may mean that IF deformation occurs it may take a lot longer to create the "lip" as in effect there ain't no material where the lip is.
    Watch this space!
     

    Attached Files:

    Last edited: May 28, 2020
    TORNADOS7, hmangphilly and smashie like this.
  2. hmangphilly

    hmangphilly Posting Addict

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    I think you;re in with a shout with that
     
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  3. Virak

    Virak Busy Member

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    I have to decide if I keep to length or reduce, tempted to reduce to same as OM FV.
    If anyone has advise on reasons not to shorten (<12ftlbs) please pm me,I just need simples working Airifle that don't eat it's FV's.
     
  4. Bezzer

    Bezzer Posting Addict

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    The shaft on the Robb one is longer is that what you are referring to? If so longer stem gives longer duration and further opening of the valve equals more power so you may have to shorten it anyway.
     
    Virak likes this.
  5. Virak

    Virak Busy Member

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    Ta seems to stick to standard makes sense-1 less variable
     
  6. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    Head design is interesting and potentially has promise IMO, but all the longer stem is going to do is reduce striker stroke and hence muzzle energy. The "longer stem = longer duration" argument is total b*llocks unfortunately.
     
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  7. hmangphilly

    hmangphilly Posting Addict

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    if the springs are kept the same , longer stem should mean shorter stroke and very possibly a drop in power .
     
    Virak likes this.
  8. Bezzer

    Bezzer Posting Addict

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    b*llocks ? Why have you tried it.? I have, I made longer stems when I first regged the 400 to reduce the stroke and the power went up. The hammer spring obviously must bottom out the OE valve stem and therefore had enough rate to push the extended stem in further despite the shorter stroke. That's the reason I kicked that idea into touch and made/used extended nose hammers instead.
     
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  9. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    I don't need to, it's simple physics. Longer stem = shorter stroke = less striker energy = shorter valve duration.

    You may be correct about the striker bottoming out and limiting valve lift / duration at the lower-than-intended operating pressures potentially encountered with a reg.. and (even more excessive striker spring preload notwithstanding) a longer stem might be a viable way of reducing striker input energy for a regged application.

    However, nothing in the OP's post suggests reg use, nor does the speil about the valve on Robb's site:

    Much like a lot of the stuff he sells, the "benefits" of his products seem to be touted on the grounds of thinly-veiled and nebulous claims of increased output rather than any tangible, quantifiable, real-world improvements.

    I stand by my original assertion - essentially that putting one of these in an unregged gun will cause the muzzle energy to fall; probably significantly.
     
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  10. hmangphilly

    hmangphilly Posting Addict

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    You would imagine they've been tested though Mr Leaf .
    So worth giving it a go and if a bit of extra hammer spring or tweaking compensates the shorter stroke then Happy days .

    Worst case , the op can reduce the stem to std dimensions .

    If he can get his gun to work as before , but not gobble a valve every 2000 shots I'm thinking that's progress .
     
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  11. Bezzer

    Bezzer Posting Addict

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    Yes you do need to, that's not simple physics, you are totally ignoring the strength and effect of the spring in your "formula"
     
    Virak likes this.
  12. cloverleaf

    cloverleaf Super Moderator Staff Member Mod/Admin

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    You'd imagine, certainly; although whether this is actually the case of course is a different matter.

    As above, the head design shows promise and as you suggest has to be a good thing if it sidesteps the effect of extrusion as seen in the standard valve.

    I still maintain that the extended stem is a pointless, ill-conceived and backwards step for an unregged application though - especially if greater striker spring preload is added to overcome the effects of its longer stem as the 400 series already runs a lot of preload which contributes towards its poor efficiency on the low-pressure side of the velocity curve.

    I am, but given the forces involved the static load generated by the striker spring is negligeable compared to the pressure loading holding the valve shut.

    In a standard rifle the striker spring exerts between approx. 30N (uncocked) and 45N (cocked) of static force on the striker. By contrast the pressure loading on the closed valve is between around 340N (120bar) and 510N (180bar), or 150N (120bar) and 225N (180bar) with the valve open..

    As such you can see that by far the dominant force here is that acting to close the valve; meaning that at these pressures the static spring loading has no chance of overcoming that on the valve and the striker needs the kinetic energy transferred to it by the spring during it's free travel to open the valve. Reducing this stroke reduces this striker's kinetic energy, which cannot possibly be recovered once the striker is in contact with the valve since the much greater force holding the valve shut limits the striker's stroke and hence energy transfer from the spring.

    As always these assertions on my part are evidence (rather than ego) led and I'm happy to be disproven by a properly conducted test (although I don't expect to be) - Perhaps @Virak could do a back-to-back test of muzzle energy over the guns' normal operating pressure range between a new, non-extruded valve and the Robb replacement (although really to be a fair test the head design and material should be identical as well, or course)..?
     
    Virak likes this.
  13. Virak

    Virak Busy Member

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    I have stripped the gun down, cleaned, polished and cut the Terry Rob firing valve down to the Sam "working length" as theO firing valve.

    On assembly I have spent some time working out why the bolt (S355) and sleeve (S345)( attached to the firing bolt and "gating into the bolt housing (S594) show damage machining.

    Firstly the bolt and sleeve have been machined down to size to apparently fit the gate.
    However the bolt was not central and thus contacting the gate the gate.
    I removed the 0 ring(S650) at end of bolt at bolt knob interface and fitted a thinner O ring.
    I cleaned up and polished the sides of the bolt and sleeve.
    I de- burred the corner of the gate
    The bolt and sleeve are 95% fit for now.
    Not sure if maybe a mylar washer or simmilar could be better fit, but the O ring has a bit of give/ spring in it.see images
    Images
    1+2) Bolt and sleeve in housing as manufactured
    3+4)BoltaBolt sleeve after polishing and thinner O ring at bolt/ knob
    5) O rings thinner is now ftted
     

    Attached Files:

  14. Virak

    Virak Busy Member

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    On assembly of the gun I noted that in the unloaded state, the hammer actually slightly loads/touches the firing pin.
    This seemed wrong, but the gun has pumped up via stirrup pump ok.
    One would have thought the hammer/spring assembly might be stronger than the firing valve.
    I was reluctant to cut the TR firing valve any different to the OE.

    QUESTION
    BUT IF the above hammer valve assy/ firing valve are in contact can it be eliminated that this has NO issue with the FV deforming/ short life.
    Logic is the firing valve is closed or open ( firing) and pressure whilst not in use has no affect ( or air would leak).
     
  15. Virak

    Virak Busy Member

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    I have re- assembled the gun NOTE I have inserted a piece of pool qué into the cylinder, I guess 1/2 to 1/3 capacity taken up.
    Question would inserting wood into the cylinder would anti -logarithimatecally! reduce the shot **** as the pressure lowers.
    There seemed a more noticeable shot count at say 170 Barr compared to 110 bar?
    Anyway I would appreciate thoughts on the attached graph.
    Other than cleaning, polishing, fitting the T Rob FV at " the same 51.18mm working length to the OE FV' I have left the gun as it was shooting with the very poor sweet spot.
    With the cylinder wood filled, I have a sweetspot of 38 shots, using my worst case scenario of 1/3 volume of unusable cylinder 38X 1.33 = 50shots.
    So I await awhile on any advise/ comments on these results.
    I guess I would like to have a more linear and sub 12ft/s sweetspot if anyone could pm me as to possibly do this.
    Oh one other thing- on the maybe 8 chrono checks thru the useful Barr range, there has ALWAYS been a linear 10 or more results at 120 barr
    this seems too consistent to be random any ideas?
    Anyway graph attached and I will offer chrono results on going to see if the TR firing valve is more resilient to the OE one(s)
     

    Attached Files:

  16. Virak

    Virak Busy Member

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    I have added to the graph the very first chrono results when the gun was new.
    The TR TV seems to be certainly a bit more stable and maybe if the TR power was lower, there may be even more linearity for comparison.
    Again as previous I would appreciate comments from your observations of the graph.
    Again currently the cylinder is about 2/3 capacity - filled with a length of snooker qué.
    TA
     
  17. Virak

    Virak Busy Member

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    For info, I had concerns on the hammer contacting the firing valve.
    As I had no further advise on the cause, I stripped the gun down.
    The TR firing valve had unscrewed a turn or so.
    I guess this happen on cleaning it prior to fitment.
    The OE fring valves I have replaced are impossible to unscrew.
    I added a tiny drop of picture on the TR firing valve thread.
     
  18. Virak

    Virak Busy Member

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    First result after 200 pellets, the results are pretty good at replicating the 1st results above ( but those had a dowel in the cylinder).
    So this will give me 45 shots all within 15ft/s
    The worst difference between shots is 7ft/s.
    Quite pleased with this, FINGERS CROSSED this Terry Rob FV does not notably deform too soon..
    The next chrono results will be with weighed pellets 160-100 Barr.
    As previous I would appreciate comments positive or negative to this work/ chrono " sweet spot"
    Thanks IMG_20200527_213156.jpg
     
    john79 likes this.
  19. Virak

    Virak Busy Member

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    Put the scope on,zeroed and shot initially 50 pellets 170-130 Barr, ended up shooting 65 shots right down to 100barr.
    This agrees( obviously) with the chrono results or 50+ shots 160-100 bar.
    Clearly there are 35 shots that one could be converted to pellet on pellet just enough for HFT.
    I will have to use a bit of holdover 160-145 barr.
    So pleased with the result, but realise maybe as Air Arms Boy I should fit a regulator just to get that linearity.
    I will post another chrono chart at about 500 pellets
     
  20. TORNADOS7

    TORNADOS7 Top Poster

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    Interesting to see what the outcome is re the valve buddy... :thumb:
     

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